The plasma coating of the hollow bodies particularly serves to lower the permeability of plastic bottles, particularly PET beverage bottles, with respect to gases such as oxygen and/or carbon dioxide. Conventional types of plastics have an inadequate gas barrier with respect to oxygen-sensitive and/or carbonated drinks, such as e.g. beer, juice, milk, so that the resulting shelf life of a product is too short. In comparison with uncoated reference bottles, coated PET bottles normally show a BIF (barrier improvement factor) of particularly 5 (oxygen) and about 2-3 (carbon dioxide), i.e., they have a barrier increased by this factor. This considerably prolongs the shelf life of a product.
A further application of plasma-coated PET bottles in the beverage industry is that contaminants and/or impurities inside the PET, such as acetaldehyde, are efficiently prevented from permeating through the layer and from passing into the interior of the bottle. Treated PET bottles can thus be used for beverages without natural flavor, such as particularly uncarbonated water.
A further advantage of coated hollow bodies is that cleaning agents and/or sterilizing agents, for instance H2O2 or soaps, cannot migrate into the wall of the hollow body and can thus not pass into the product after the hollow bodies have been filled. A further advantage is that organic filling materials/impurities do not migrate into the bottle wall, thereby permitting a re-use of the hollow bodies, e.g. for food packaging.
Apart from plasma coating, plasma sterilization is particularly possible for cleaning hollow bodies, particularly in the food or beverage packaging industry. Sterilization of the hollow bodies by means of plasma is carried out through the action of shortwave electromagnetic radiation in the UV range, as well as ionized and radical plasma components, on contaminants, such as harmful microorganisms. Plasma sterilization can be carried out as an alternative to plasma coating. Both methods can also be carried out in combination during treatment of a hollow body.
Apparatuses and methods for the plasma treatment of containers, particularly for the manufacture of plasma-coated plastic plasma containers for improving the gas barrier are generally known.
DE 102 25 659 A1 shows methods in which the interior of a workpiece is subjected in an evacuable individual chamber to plasma coating, wherein the plasma is generated by using microwave energy.
A further method for the inner coating of particularly PET bottles is described in US 2002/1,796,031 A1. A carbon-containing coating is here applied to the inside of the bottles via a plasma method in which the bottle to be coated is introduced into an individual chamber and a plasma is generated by means of microwave energy in the interior of the bottle.
The methods disclosed in the prior art have the drawback that the plasma treatment of the hollow bodies is carried out in individual chambers in which a respective hollow body is positioned. In the case of a large-scale implementation where a great number of coated containers are to be produced within a period of time as short as possible, this requires a great number of individual chambers of such types. As a result, such apparatuses and methods known from the prior art entail great costs and require large facility dimensions.
An approach for circumventing such drawbacks is described in DE 10 2004 03 6063 A1. Disclosed are there an apparatus and a method for plasma coating and/or sterilization in which in a vacuum chamber a plasma is generated by means of microwave energy in a vacuum chamber through a rod-shaped electrode in the interior of containers.
Microwave-generated plasmas, i.e. plasmas generated by electromagnetic waves in the GHz range, have in general the drawback that it is difficult to achieve a uniform plasma treatment of the substrates, particularly in the case of hollow bodies. This means that it is particularly difficult to guarantee the deposition of a uniform layer having a strong gas barrier. Furthermore, a high energy input is needed in the case of microwave-generated plasmas, which is disadvantageous in terms of costs.
To circumvent these drawbacks, there are proposals that the plasma should be generated by applying high-frequency energy, i.e. electromagnetic waves, in the kHz to MHz range. This is e.g. described in U.S. Pat. Nos. 6,180,191 B1, 6,539,890 B1, or 6,112,695. However, in some instances there is also described an individual chamber in which the containers are positioned during plasma treatment.
Hence, the apparatuses and methods described in the prior art are complicated, expensive or only suited to some extent for a large-scale process because they entail high costs, large dimensions and low efficiency as far as the facility is concerned.